Expansible bolt anchoring devices are widely used for attaching articles to, for example, the surfaces of masonry or brick walls or structures. A typical anchoring device comprises a segmented right cylindrical shell, and an expansion member located in the shell. The expansion member may be in the form of a nut mounted on the bolt (a "loose bolt" anchor) or may form the head of the bolt (a "projecting bolt" anchor). The expansion member is initially positioned at the inner end of the segmented shell such that rotation of the bolt, or rotation of a nut on the shank of the bolt, will draw the expansion member into the shell and radially expand the shell. Thus, when the device is located in a hole of corresponding diameter to the segmented shell, tightening the bolt or nut expands the shell radially to engage the hole wall.
Tightening of the bolt or nut in the desired "one-handed" manner requires that the shell engages the hole wall and is held against rotation in the hole. This requires that the shell is a snug fit in the hole, which is often difficult to achieve. In the absence of such a close fit, the shell must be held or jammed in the hole by some other means, such as part-filling the hole with packing material.
The three or four segments which typically form the shell are conventionally formed of cast or pressed metal. The segments are held together in the desired cylindrical form around the bolt by a ferrule placed over one end of the segments. A spring clip also encircles the segments intermediate their ends to prevent the shell segments from falling apart. Not surprisingly, the assembly of expansible bolt anchoring devices is a relatively difficult and time-consuming task and is still, in the main, a manual operation.
The majority of segments utilized in existing anchoring devices are relatively rigid and are only subject to minor deformation on the bolt or nut being tightened to expand the shell to engage the segments with the hole wall. Accordingly, the segments tend to engage the hole wall with a point or line contact, such that pressure forces experienced by the hole wall are relatively high and tend to cause a degree of crushing of the masonry or brickwork. A limited degree of such crushing may be advantageous, serving to anchor the shell securely in the hole. However, in softer materials the surrounding stone may crumble to such an extent that the device will not remain in the surface, requiring drilling of a larger hole and use of a larger diameter device.
Numerous proposals have been made to provide bolt anchoring devices with solid resilient sleeves in place of the conventional segmented metal sleeves. Devices of this type are described in GB-A-685643, GB-A-1530344, GB-A-774002, GB-A-886400, EP-A-0494741, EP-A-0105829 and U.S. Pat. No. 4472088. However, these devices have not been adopted by the industry and the resilient sleeve anchor bolts currently available on the market are provided for non-load bearing applications only.